1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to a large relative aperture, high range zoom lens of the rear focus type whose range is 6-8 and whose F-number is 1.8-2.0 or thereabout, to be used with photographic cameras, video cameras, or broadcasting cameras.
2. Description of the Related Art
In the zoom lenses to be used with photographic cameras or video cameras, the so-called rear focus type is variously proposed in which a lens unit other than the front or first lens unit is moved to effect focusing.
In general, compared with a zoom lens whose front lens unit is movable for focusing, the zoom lens of the rear focus type has the advantage that the effective diameter of the front lens unit becomes small, contributing to a reduction of the bulk and size of the whole lens system, and the advantage that close-up photography, especially, ultraclose-up photography, becomes easy to carry out, and further the advantage that, since a focusing lens unit is relatively small in size and light in weight and a small driving power therefore is sufficient, rapid focusing is possible.
Of the zoom lenses of such a rear focus type, the so-called 4-unit zoom lens disclosed in Japanese Laid-Open Patent Application No. Sho 63-44614, for example, comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of negative refractive power for compensating for the image shift with variation of the focal length, and a fourth lens unit of positive refractive power, wherein focusing is performed by moving the third lens unit. However, this zoom lens tends to increase in the physical length, because some space must be taken to allow the third lens unit to move.
In Japanese Laid-Open Patent Application No. Sho 58-136012, a zoom section is constructed with three or more lens units, one of which is moved for focusing.
In Japanese Laid-Open Patent Application No. Sho 62-247316, four lens units are used, of which the first, when counted from the object side, is of positive refractive power, the second is of negative refractive power, the third is of positive refractive power and the fourth is of positive refractive power, wherein the second lens unit is moved to vary the focal length, while the fourth lens unit is simultaneously moved to compensate for the image shift, and focusing is performed also by the fourth lens unit.
Meanwhile, in recent years, as video cameras for public use have widespread, there are strong demands for even their lenses to be reduced in size and weight and for their price to be lowered. To meet these demands, it is highly recommended not only to employ the aforesaid rear focus type in the zoom configuration, but also to introduce an aspherical lens to reduce the total number of constituent lens elements. In Japanese Laid-Open Patent Application No. Hei 1-201614, in a 4-unit zoom lens of the rear focus type, aspheric surfaces are introduced into the second to fourth lens units, so that a reduction of the number of lens elements is achieved.
Also, in Japanese Laid-Open Patent Application No. Hei 2-39011, in another 4-unit zoom lens, aspheric surfaces are introduced into the first, third and fourth lens units, so that a reduction of the number of lens elements is achieved.
As described before, the use of the rear focus type in the zoom lens design leads to the possibility of minimizing the bulk and size of the entirety of the lens system and further to the extension of the focusing range to shorter object distances.
On the other hand, however, the variation of aberrations with focusing is increased so largely as to give rise to a very difficult problem of simultaneously fulfilling the requirements of minimizing the size of the entire lens system and of maintaining a high optical performance for an infinitely distant object to an object at the minimum distance, or throughout the entire focusing range.
Particularly, in a large relative aperture, high range zoom lens, the problem of maintaining good stability of high optical performance throughout the entire zooming range as well as the entire focusing range becomes very serious.
It has been known in the art to make aspherical some of the lens surfaces so that the number of constituent lens elements is reduced and the various aberrations are well corrected. With not only the zoom type but also many other types of photographic systems, such an aspheric lens surface has been adopted in various ways. The mere replacement of the spherical surface by an aspherical surface, however, does not insure that the optical system takes a much simplified form, and does not remarkably diminish the difficulty of correcting various aberrations for high optical performance.